196 



INDICATOR OCULARS 



[Ch. VI 



as the ocular micrometer, and the pointer like the micrometer is 

 magnified with the real image and appears as a part of the projected 

 image (fig. 117 B). By rotating the ocular or the pointer any part 

 of the real image may be pointed out as one uses a pointer on a wall 

 or blackboard diagram. By means of the indicator eye-piece one 



Fig. 118 A. Ring around One of the Sections of a Series for Demonstrat- 

 ing some Organ Especially Well. 



Fig. 118 B A Microscopic Preparation with a Ring around a Small Part 

 to Show the Position of Some Structural Feature. 



can be certain that the student sees the desired object, and is not 

 confused by the multitude of other things present in the field. This 

 device has been invented many times. It illustrates well the adage: 

 "Necessity is the mother of invention," for what teacher has not 

 been in despair many times when trying to make a student see a 

 definite object and neglect the numerous other objects in the field ? 

 So far as the writer has been able to learn, Quekett was the first to 

 introduce an indicator ocular with a metal pointer which was ad- 

 justable and could be turned to any part of the field or wholly out 

 of the field. 



It is not known who adopted the simple device of putting a fine 

 hair on the diaphragm of the ocular, as shown in fig. 117. This may 

 be done with any ocular, positive or negative. One may use a little 

 mucilage, Canada balsam, or any other cement to stick the hair on 

 the upper face of the diaphragm so that it projects about halfway 

 across the opening. When the eye-lens of the Huygenian ocular is 

 screwed back in place the hair should be in focus. If it is not, screw 

 the eye-lens out a little and look again. If it is not now sharp, the 

 hair is a little too high and should be depressed a little. If it is less 

 distinct on screwing out the ocular it is too low and should be 



